Is there any “rule” (or rule-of-thumb) for the solder (e.g. lowest or
highest karat) to use when joining 18kt to 14kt or other
combinations. I’m fabricating not repairing but, I would also be
interested for repair. I asked over in RCJ (during Hanuman’s holiday)
but, no one offered an answer, and thought, perhaps there is no
positive answer. tx, Regis

Regis, use the highest temp solder for the alloy with the lowest
melting point. So, If you were soldering 18 k white gold to 14 k
yellow you it would be logical to use 14 k yellow hard first. Check
the flow point of 18 k white solders too. It may be possible to use
18 k or 14 k white easy if you are confident in your soldering
ability enough that you won’t melt the yellow. 14 k yellow flows
around 1585 F depending on the alloy. 14 k hard is around 1490-1485
depending on whether it is plumb or not. 18 k white easy flows at
1510 and 14k white easy flows around 1325. Then you can move down to
the lower melting point solders as needed. You will need to know the
melt points of all the alloys and solders you deal with. Many
catalogs have this